A window ball grid array (WBGA) package utilizes a substrate having a through opening where electrically-connecting bonding wires are received so as to reduce the package size. FIGS. 2A–2F show the procedural steps of a fabrication method of the WBGA semiconductor package. As shown in FIG. 2A (cross-sectional view) and FIG. 2B (top view), once the substrate 10 having the through opening 102 is prepared, at least one semiconductor chip 11 is mounted on an upper surface 100 of the substrate 10 and over the opening 102 by means of an adhesive 12. The adhesive 12 is usually applied along two relatively longer sides of the opening 102, with gaps G along two relatively shorter sides of the opening 102 formed between the chip 11 and the substrate 10 and not applied with the adhesive 12. A plurality of bonding wires 13 are formed through the opening 102 to electrically connect the chip 11 to a lower surface 101 of the substrate 10.
As shown in FIGS. 2C and 2D, the substrate 10 with the chip 11 and the bonding wires 13 is placed and clamped between an upper mold 17 and a lower mold 18 of an encapsulation mold, wherein the chip 11 is received in an upwardly recessed cavity 170 of the upper mold 17, and the bonding wires 13 are received in a downwardly recessed cavity 180 of the lower mold 18. A first molding process is performed to inject a conventional resin material (such as epoxy resin) into the downwardly recessed cavity 180 of the lower mold 18 to form a lower encapsulation body 15 for filling the opening 102 and encapsulating the bonding wires 13; however, the gaps G between the chip 11 and the substrate 10 usually can not be completely filled by the resin material.
Then as shown in FIG. 2E, a second molding process is performed to inject the resin material into the upwardly recessed cavity 170 of the upper mold 17 to form an upper encapsulation body 14 for encapsulating the chip 11.
After the first and second molding processes are complete, the upper and lower molds 17, 18 are removed from the substrate 10, such that area on the lower surface 101 of the substrate 10 not covered by the lower encapsulation bodies 15, such as ball pads 103, can be exposed outside. Finally as shown FIG. 2F, a plurality of solder balls 16 are bonded to the exposed ball pads 103 on the lower surface 101 of the substrate 10, and the WBGA semiconductor package is fabricated.
However, the above package fabrication method leads to significant drawbacks. First, the downwardly recessed cavity of the lower mold needs to be sized in accordance with the size of the substrate opening to allow the lower encapsulation body to completely cover the opening but not occupy area or ball pads on the lower surface of the substrate. Therefore, when using substrates having openings of different sizes, new lower molds having correspondingly-dimensioned downwardly recessed cavities are required, making the fabrication cost undesirably increased. Moreover, two-stage encapsulation is required, including the first molding process and the second molding process. This two-stage encapsulation process not only complicates the fabrication performance but also leads to a resin-flash problem. During the first encapsulation process for forming the lower encapsulation body, area on the lower surface of the substrate around the opening and underneath the chip usually lacks firm support from the upper mold and is not strongly clamped by the encapsulation mold, such that the resin material injected into the downwardly recessed cavity of the lower mold may easily leak or flash through the interface between the lower mold and the substrate to unintended area the lower surface of the substrate. If the ball pads are contaminated by the resin flash, they can not be well bonded to solder balls, thereby degrading reliability of the semiconductor package. Besides, since the gaps between the chip and the substrate are usually not completely filled by the resin material, voids in the gaps undesirably cause popcorn effect and damage the package structure.
In order to solve the above problems, another fabrication method of a WBGA package is proposed in a U.S. patent application filed on Sep. 24, 2003 by the present applicant. As shown in FIG. 3, after the above substrate 10 is formed with the chip 11 and bonding wires 13, a spacer 19 is attached to the lower surface 101 of the substrate 10. A through hole 190 is formed through the spacer 19 corresponding to the opening 102 of the substrate 10 and accommodates wire loops of the bonding wires 13. A single molding process is performed. The substrate 10 and the spacer 19 are clamped between the upper mold 17 and a lower mold 18′ having a flat top surface 181 in contact with the spacer 19. The resin material is injected into the upwardly recessed cavity 170 for encapsulating the chip 11 and also flows through the gaps (not shown) between the chip 11 and the substrate 10 to the opening 102 and the through hole 190 so as to encapsulate the bonding wires 13 and fill the opening 102, through hole 190 and gaps (not shown). The molds 17, 18′ and the spacer 19 are removed from the substrate 10 after molding, such that the chip 11 and bonding wires 13 are encapsulated by an integral encapsulation body 1.
The above fabrication method beneficially utilizes the spacer. The spacer is cost-effective to fabricate and can be formed with variously-sized through hole in accordance with the substrate opening without significantly increasing the fabrication cost. The gaps between the chip and the substrate are completely filled with the resin material, thereby not producing voids or popcorn effect. The chip and bonding wires are encapsulated by an integral encapsulation body formed by a single molding process, not complicating the fabrication performance. And the spacer clamped between the substrate and the lower mold supports area the lower surface of the substrate conventionally lacking firm support from the upper mold and thus helps eliminate the above resin flash through the interface between the lower mold and the substrate. However, the resin flash may still possibly or accidentally occur through the interface between the spacer and the substrate and contaminates unintended area such as ball pads on the lower surface of the substrate.
Therefore, the problem to be solved herein is to provide a fabrication method of WBGA semiconductor package to effectively solve the resin-flash problem.